This research will determine the fraction of the mononuclear phagocytic system (MPS) that is accessible to liposomes in vivo. Liposomes are cleared from circulation primarily by cells of the MPS and it is presumed that blood monocytes avidly take up liposomes. However there is little direct evidence to support this notion. We will determine the pharmacokinetics and distribution of liposome encapsulated agents in mice and will specifically quantitate the fraction of the liposome dose that is taken up by blood monocytes. This will be measured by eliciting the monocytes to migrate into the peritoneal cavity or another site of inflammation after they have internalized the liposome. As a control, monocytes will be depleted from mice using 89Sr. In the monocyte depleted state there is no migration into sites of inflammation. Therefore monocyte mediated liposome disposition cannot be confused with liposome extravasation. To maximize the extent of liposomal drug delivery to the blood monocyte, ligands such as mannosylphosphate that can interact with cell surface receptors or antibodies that recognize monocyte surface determinants will be incorporated into the liposome. In addition to increase the fraction of the liposome contents that are delivered into the cytoplasm we will employ novel pH sensitive liposomes that undergo phase transitions at pH 5 which makes them fusion competent. The factors that control the cellular availability from the pH sensitive liposomes in vivo will be defined using a double radioactive label to follow the fate of the liposome contents. The pharmacological availability of liposome contents will also be measured in the monocytes. This will be done using liposome encapsulated anti-sense oligonucleotides to specifically inhibit inflammatory mediators such as interleukin 1 and tumor necrosis factor induced by lipopolysaccharide. Target sequences in the start codon of the murine interleukin 1 alpha have been identified and antisense oligonucleotides will be synthesized and delivered in liposomes to inhibit interleukin 1 alpha synthesis. The tools for examining the question of cellular availability of liposome encapsulated agents should be useful for establishing a pharmacological dose response relationship in cells of the MPS. Success in achieving the objectives of this proposal will permit a more complete understanding of liposome mediated drug delivery. Based upon the information generated in the early studies of the research we will establish the factors that control cytoplasmic delivery and use this information to design fusogenic peptides that can enhance the efficiency of cytoplasmic delivery from liposomes into cells. Implementation of the concepts of membrane destabilization and pH sensitive drug delivery may lead to novel methods to transfer macromolecules into endocytotically active cells in vivo.